It is essential for the efficiency of heat pipes of the type contemplated that the largest possible amount of condensed medium be fed back through the capillary system, from the heat emitting end of the heat pipe to its heat absorbing end. To achieve this purpose, the effective diameter of the capillaries should be as small as possible, and as many capillaries as possible should be present. To accommodate as many capillaries as possible, the capillary system should have the greatest possible pore volume. The material of which the capillary system consists must be readily wettable by the volatile medium, since this factor is decisive in forming the capillary fluid column. Furthermore, the capillary system should consist of a material with good heat conductivity. This is necessary in order to secure rapid transfer of the applied heat to the volatile medium at the heat absorbing end of the heat pipe, and rapid transfer of the medium's heat to the wall of the heat pipe at its heat emitting end. Finally, it would be desirable for the capillary system to be very temperature resistant, so that liquids with high boiling points can be used, such as, for example, liquid metals like cadmium, cesium, sodium, zinc, and the like. This is particularly important when such heat pipes are used at temperatures of the order of 2000.degree. C.
Known capillary systems meet the above requirements only to a very inadequate extent. Capillaries in the form of lengthwise grooves along the inner wall of the heat pipe can in practice be made only with relatively large dimensions, so that the capillary force is correspondingly small. Capillary systems of sintered metal power have large mass and a correspondingly small pore volume, which lies below 50%.